LAUSR

Simple Summary Approximately 60% of all melanoma patients have cancer with an activating BRAF mutation that can be effectively treated with a BRAF inhibitor. Unfortunately, such treatment often leads to rapid development of BRAF inhibitor-resistance. These BRAF inhibitor-resistant melanomas are also characterized by increased progression and metastasis. The present study was undertaken to identify an effective antimetastatic therapy for these patients. We found increased activity of the cytoskeletal-associated MARCKS protein in BRAF inhibitor-resistant melanoma cells compared to BRAF inhibitor-sensitive melanoma cells and explored the mechanism behind this increased activity. We also showed that inhibition of MARCKS activity not only reduced the invasion and migration of BRAF inhibitor-resistance melanoma cells, but also their metastatic capacity. Our study reveals that MARCKS is a critical mediator of enhanced metastasis in BRAF inhibitor-resistant melanoma cells and thus a reasonable, novel antimetastatic therapeutic target in patients with BRAF inhibitor-resistant melanoma. Abstract Treatment of melanoma with a BRAF inhibitor (BRAFi) frequently initiates development of BRAFi resistance, leading to increased tumor progression and metastasis. Previously, we showed that combined inhibition of elevated WNT5A and IL-6 signaling reduced the invasion and migration of BRAFi-resistant (BRAFi-R) melanoma cells. However, the use of a combined approach per se and the need for high inhibitor concentrations to achieve this effect indicate a need for an alternative and single target. One such target could be myristoylated alanine-rich C-kinase substrate (MARCKS), a downstream target of WNT5A in BRAFi-sensitive melanoma cells. Our results revealed that MARCKS protein expression and activity are significantly elevated in PLX4032 and PLX4720 BRAFi-R A375 and HTB63 melanoma cells. Surprisingly, neither WNT5A nor IL-6 contributed to the increases in MARCKS expression and activity in BRAFi-R melanoma cells, unlike in BRAFi-sensitive melanoma cells. However, despite the above findings, our functional validation experiments revealed that MARCKS is essential for the increased metastatic behavior of BRAFi-R melanoma cells. Knockdown of MARCKS in BRAFi-R melanoma cells caused reductions in the F-actin content and the number of filopodia-like protrusions, explaining the impaired migration, invasion and metastasis of these cells observed in vitro and in an in vivo zebrafish model. In our search for an alternative explanation for the increased activity of MARCKS in BRAFi-R melanoma cells, we found elevated basal activities of PKCα, PKCε, PKCι, and RhoA. Interestingly, combined inhibition of basal PKC and RhoA effectively impaired MARCKS activity in BRAFi-R melanoma cells. Our results reveal that MARCKS is an attractive single antimetastatic target in BRAFi-R melanoma cells.